This invention relates generally to circuits for isolating various electrical signals and more particularly to an improved isolation circuit employing a pulse transformer.
Various applications in the electronics industry require isolation of an electrical signal. One such application, for example, is that of off-line power supplies. In such power supplies, it is necessary to convey information about the status of the output voltage to a power controller. This information is commonly known as an error voltage signal or simply an error signal. More generally, this signal may be characterized as an analog signal. In many power supplies, it is required that the output voltage be electrically isolated from the source of power. If this requirement exists and, if, as is often the case, the power controller is electrically connected to the source of power, then it is necessary that the error signal be conveyed across an electrical isolation boundary. In such a process, the signal to be conveyed across the isolation boundary is known as the source signal, and the reconstructed conveyed signal is known as the target signal.
Two popular methods for conveying an analog signal across an electrical isolation boundary are known in the prior art. One such method is by optical conveyance using an optical isolator, also called an opto-coupler. Another such method is by magnetic conveyance using a transformer, which serves in the more specific capacity in such an application as a pulse transformer. Prior art power supplies which use primary-side controllers typically employ one of these two methods to isolate the error signal.
An opto-coupler is commonly employed in that application because it is easily understood, it does not require a clocking signal, and it is easily configured with a single three-pin TL431 integrated circuit in combination with several passive components to perform the error signal isolation. There are some limitations in using an opto-coupler in such an application. These limitations include, but are not limited to, the following:
1) the initial and long term gain stability of the opto-coupler degrade the capability for the closed-loop response of the power supply to be optimized;
2) the isolation voltage is limited by the structure of the opto-coupler;
3) the isolation capacitance of the opto-coupler can be problematic in applications sensitive to AC isolation (e.g. medical equipment); and
4) the simplest configuration of an opto-coupler with a TL431 device produces a unity-gain zero in the error amplifier which complicates optimization of the closed-loop response of the power supply.
A pulse transformer may alternatively be employed in that application. It, too, has limitations inherent in the prior art means of implementation. These include, but are not limited to, the following:
1) it requires more complex supporting circuitry than the opto-coupler technique;
2) it requires a clocking signal on the electrical side in common with the source signal;
3) it can introduce spectral interference into the power supply system if the pulses driving the pulse transformer are not synchronized with the switching of the power transformer of the power supply; and
4) the simplest means for recovering the amplitude information of the isolated signal (i.e. by single-ended rectification) has a non-linear performance which is difficult to characterize.
Both of the prior art means of electrically isolating a signal have one common and more fundamental limitation. The limitation is inherent in what can be described as a send and receive process employed by both means. The action of transmitting or sending the signal is performed by the portion of circuitry which is electrically isolated from the portion of circuitry which receives the signal to perform its function. The portion of circuitry which receives the signal requires a source of voltage to perform its function. The portion of the circuitry which sends the signal also requires a source of voltage to accomplish the transmitting process. Thus, in the aforementioned example of a power supply, the portion of circuitry electrically connected to the output of the power supply, commonly known as the secondary side, requires a source of voltage in order to transmit the error signal to the primary-side controller. Often, the output voltage of a power supply is used to power the sending circuitry. When the output voltage is not fully established, such as when the supply is initially turned on or when it is overloaded, there may be an insufficient source of voltage to accomplish the transmitting process. This leaves the options that either the power supply must provide a supplemental isolated source of voltage to power the sending circuitry on the secondary side or the primary-side controller circuitry must operate apart from having the error signal information or any other information about the output side of the power supply other than that which may be derived through its main power transformer. The former option is often not economically feasible and the latter option disallows a well-controlled current fold-back characteristic.
It is therefore a principal object of the present invention to provide a signal isolator circuit that overcomes the limitations set forth in the preceding paragraphs, but which nevertheless employs a pulse transformer in order to eliminate the disadvantages of prior art opto-couplers.
This and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by employing a retrieval process, as opposed to the prior art pulse transformer and opto-coupler designs which employ a send and receive process.
In summary, the retrieval process of the present invention is accomplished in a higher accuracy configuration by repetitively, in synchronization with a clocking signal, performing the following steps on the electrical side of the pulse transformer where the target signal must be derived:
1) drive a current pulse into the pulse transformer;
2) sample (detect) the voltage across the pulse transformer after it has stabilized but while the current pulse is still active;
3) hold (store) the sampled voltage for a full clock cycle; and, if necessary,
4) filter the sampled voltage signal.
On the other electrical side of the pulse transformer, where the source signal has its origin, the retrieval circuitry is configured such that when the pulse transformer is being driven by the current pulse, the voltage will be clamped by the source signal.
Furthermore, both electrical side of the pulse transformer must be configured to allow a reversal of the voltage polarity. Either side of the pulse transformer must also be configured to provide a reset path for the magnetizing current.
It should be noted that the sampling and holding requirement may also be accomplished by passive means, rather than active means required by sample-and-hold circuitry, by using a peak detection circuit consisting of some rectification means and a capacitor for holding the voltage of the recovered target signal. This will create a further bandwidth limitation and will also exhibit the non-linearities associated with the simplest means of recovering amplitude information in the prior art pulse transformer signal isolator circuit.
In a lower accuracy configuration, the retrieval process of the present invention involves the same steps as set forth above with the exception that, rather than holding the sampled Voltage across the pulse transformer, the reflected impedance is detected as being either high or low, by some quantitative measure, attainable from the sampled voltage, and the detector output will be set to either a high or low voltage, corresponding to the detected impedance. This simpler configuration is sufficient to accomplish the isolation of digital signals, since precise analog information is not needed.
Since the retrieval process is initiated and powered by circuitry on the target signal side of the electrical isolation boundary, the limitations associated with the prior art pulse transformer means of electrically isolating signals are overcome as follows:
1) the retrieval circuitry is not particularly complex;
2) the clocking signal can be shared with the primary-side controller since the clocking is required only on the target signal side;
3) spectral interference is inherently eliminated by virtue of synchronization of the retriever circuit with the primary-side controller; and
4) the sample-and-hold circuitry eliminates non-linear performance characteristics.
A further advantage is realized in that the portion of the retriever circuitry which is electrically connected to the primary-side of the pulse transformer can be integrated into a primary-side control integrated circuit with the addition of only a single pin.